Cisco Catalyst 2950 Software Guide

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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 9 Configuring STP
Understanding Advanced STP Features
Understanding Root Guard
The network of a service provider (SP) can include many connections to switches that are not owned by
the SP. In such a topology, STP can reconfigure itself and select a customer switch as the STP root
switch, as shown in Figure 9-13. You can avoid this situation by configuring the root-guard feature on
interfaces that connect to switches outside of your customer’s network. If STP calculations cause an
interface in the customer network to be selected as the root port, root guard then places the interface in
the root-inconsistent (blocked) state to prevent the customer’s switch from becoming the root switch or
being in the path to the root.
If a switch outside the network becomes the root switch, the interface is blocked (root-inconsistent state),
and STP selects a new root switch. The customer’s switch does not become the root switch and is not in
thepathtotheroot.
Caution Misuse of the root-guard feature can cause a loss of connectivity.
Figure 9-13 STP in a Service Provider Network
Customer network
Potential
STP root without
root guard enabled
Enable the root-guard feature
on these interfaces to prevent
switches in the customer
network from becoming
the root switch or being
in the path to the root.
Desired
root switch
Service-provider network
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Cisco Catalyst 2950 Specifications

General

Forwarding Bandwidth	8.8 Gbps
Switching Capacity	13.6 Gbps
Forwarding Rate	6.6 Mpps
Weight	3.6 kg
RAM	16 MB
Flash Memory	8 MB
Operating Humidity	10% to 85% non-condensing
Uplink Ports	2 x 10/100/1000Base-T
Dimensions	4.4 cm x 44.5 cm x 24.2 cm
Remote Management Protocol	SNMP, Telnet, HTTP
Features	Quality of Service (QoS), VLAN support
Compliant Standards	IEEE 802.3, IEEE 802.3u, IEEE 802.1D, IEEE 802.1Q, IEEE 802.1p
Status Indicators	system
Operating Temperature	0 to 45°C
Ports	24 x 10/100 Ethernet ports
MAC Address Table Size	8, 192 entries
Power Supply	Internal 100-240V AC, 50-60Hz